An inverter is an electrical device enabling conversion of DC (direct current) power from a DC power source to AC (alternating current) power. Herein ‘inverter’ generally refers to an electronic device or circuitry that is able to convert direct current to alternating current. An example of the inverter is a semiconductor bridge implemented by means of controllable semiconductor switches, such as IGBTs (Insulated-gate Bipolar Transistor) or FETs (Field-Effect Transistor), which are controlled according to a modulation or control scheme used.
One example of an electric system comprising an inverter is a photovoltaic system, such as a photovoltaic power plant or generator, in which one or more photovoltaic panels supply DC power to the inverter which converts the DC power to AC power, which may further be supplied to various AC loads via an AC network, for example. Large photovoltaic power plants may comprise a plurality of parallel inverters each receiving DC power from an array of photovoltaic panels.
In such photovoltaic power generation systems, the operation point of the PV panels is preferably driven to the best possible operation point, i.e. to a point in which a maximum amount of power can be extracted from the PV panels. This kind of technique is generally called maximum power point tracking (MPPT). As an example, the most common MPPT method is a perturb & observe (P&O) method and its variations. In this method, the MPPT voltage reference of the inverter is constantly changed, and the resulting change in the generated power is determined. On the basis of the change of power and the change of the reference value, it can then be determined whether the reference value should be decreased or increased in order to increase the amount of power extracted.
The modulation method of an inverter of such photovoltaic system may be based on a compromise between losses, chosen topology, and output current quality. The output AC current quality depends not only on the modulation method and a possible network filter but also on the AC network itself. If the AC network is rigid, the quality of the output AC current can be kept adequate with fewer commutations. The quality of the output AC current fed to the AC network may be quantified by the total harmonic distortion (THD) of the AC current and/or by one or more individual harmonics of the AC current, for example. A minimum level for the quality of the output AC current fed to the AC network is typically predetermined i.e. the harmonic content of the AC current supplied from the inverter to the AC network always needs to be below a set maximum level.
A problem related to the above solution is that the operation of the inverter may be too limited and does not enable an optimum power output.